Magnetic structures and quadratic magnetoelectric effect in LiNiPO$_4$ beyond 30T

TL;DR

This study investigates the magnetic structures and quadratic magnetoelectric effects in LiNiPO₄ under high magnetic fields up to 56T, revealing new phase transitions and a field-induced electric polarization linked to lattice symmetry changes.

Contribution

The paper reports the discovery of three new magnetic phase transitions in LiNiPO₄ above 30T and identifies a quadratic magnetoelectric effect in the high-field phase, expanding understanding of magnetoelectric coupling.

Findings

Three new magnetic phase transitions at 37.6, 39.4, and 54T.

Identification of magnetic ordering vectors in high-field phases.

Observation of a quadratic magnetoelectric effect in the high-field phase.

Abstract

Neutron diffraction with static and pulsed magnetic fields is used to directly probe the magnetic structures in LiNiPO$_4$ up to 25T and 42T, respectively. By combining these results with magnetometry and electric polarization measurements under pulsed fields, the magnetic and magnetoelectric phases are investigated up to 56T applied along the easy $c$-axis. In addition to the already known transitions at lower fields, three new ones are reported at 37.6, 39.4 and 54T. Ordering vectors are identified with ${\bf Q}_{\mathrm{VI}}$ = (0, 1/3, 0) in the interval 37.6--39.4T and ${\bf Q}_{\mathrm{VII}}$ = (0, 0, 0) in the interval 39.4-54T. A quadratic magnetoelectric effect is discovered in the ${\bf Q}_{\mathrm{VII}}$ = (0, 0, 0) phase and the field-dependence of the induced electric polarization is described using a simple mean-field model. The observed magnetic structure and magnetoelectric tensor elements point to a change in the lattice symmetry in this phase. We speculate on the possible physical mechanism responsible for the magnetoelectric effect in LiNiPO4.